Survey of the Solar System

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Survey of the Solar System

• Arny, 3rd Edition, Chapter 7

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Introduction

• The Solar System is occupied by a diversity of
  objects, but shows an underlying order in their
  movements
• The Solar System is also ordered in that the
  planets form two main families solid rocky inner
  planets and gaseous/liquid outer planets
• From observations, astronomers believe the Solar
  System formed some 4.5 billion years ago out of
  the collapse of a huge cloud of gas and dust

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Components of the Solar System

• The Sun
• The Sun is a star, a ball of incandescent gas
  whose output is generated by nuclear reactions in
  its core
• Composed mainly of hydrogen (71) and helium
  (27), it also contains traces of nearly all the
  other chemical elements
• It is the most massive object in the Solar System
  700 times the mass of the rest of the Solar
  System combined
• Its large mass provides the gravitational force
  to hold all the Solar System bodies in their
  orbital patterns around the Sun

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Components of the Solar System

• The planets
• Planets shine primarily by reflected sunlight
• Orbits are almost circular lying in nearly the
  same plane Pluto is the exception with a high
  (17) inclination of its orbit
• All the planets travel counterclockwise around
  the Sun (as seen from high above the Earths
  north pole)
• Six planets rotate counterclockwise Venus
  rotates clockwise (retrograde rotation), and
  Uranus and Pluto appear to rotate on their sides

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Components of the Solar System

• Two types of planets
• Inner planets
• Mercury, Venus, Earth, Mars
• Small rocky (mainly silicon and oxygen) bodies
  with relatively thin or no atmospheres
• Also referred to as terrestrial planets
• Outer planets
• Jupiter, Saturn, Uranus, Neptune, and Pluto
• Gaseous, liquid, or icy (H2O, CO2, CH4, NH3)
• Excluding Pluto, also referred to as Jovian
  planets
• Jovian planets are much larger than terrestrial
  planets and do not have a well-defined surface

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Components of the Solar System

• Satellites
• The number of planetary satellites has changed
  frequently over the last several years the total
  count as of August 2002 is 101 and is broken down
  as follows Jupiter 39, Saturn 30, Uranus 20,
  Neptune 8, Mars 2, Earth and Pluto 1 each, and
  Mercury and Venus are moonless
• The moons generally follow approximately circular
  orbits that are roughly in the planets
  equatorial plane, thus resembling miniature solar
  systems

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Components of the Solar System

• Asteroids and comets
• Their composition and size
• Asteroids are rocky or metallic bodies ranging in
  size from a few meters to 1000 km across (about
  1/10 the Earths diameter)
• Comets are icy bodies about 10 km or less across
  that can grow very long tails of gas and dust as
  they near the Sun and are vaporized by its heat
• Their location within Solar System
• Most asteroids are in asteroid belt between Mars
  and Jupiter indicating that these asteroids are
  the failed building-blocks of a planet
• Most comets orbit the Sun far beyond Pluto in the
  Oort cloud, a spherical shell extending from
  40,000 to 100,000 AU from the Sun
• Some comets may also come from a disk-like swarm
  of icy objects that lies beyond Neptune and
  extends to perhaps 1000 AU, a region called the
  Kuiper Belt

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Components of the Solar System

• Composition differences between the inner and
  outer planets
• Since the inner and outer planets differ
  dramatically in composition, it is important to
  understand how composition is determined
• A planets reflection spectrum can reveal a
  planets atmospheric contents and the nature of
  surface rocks
• Seismic activity has only been measured on Earth
  for the purposes of determining interior
  composition
• Density as a measure of a planets composition
• A planets average density is determined by
  dividing a planets mass by its volume
• Mass determined from Keplers modified third law
• Volume derived from a planets measured radius

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Components of the Solar System

• Density as a measure of a planets composition
  (continued)
• Once average density known, the following factors
  are taken into account to determine a planets
  interior composition and structure
• Densities of abundant, candidate materials
• Variation of these densities as a result of
  compression due to gravity
• Surface composition determined from reflection
  spectra
• Material separation by density differentiation
• Mathematical analysis of equatorial bulges

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Components of the Solar System

• Density as a measure of a planets composition
  (continued)
• This analysis of composition and structure
  reveals the following
• The terrestrial planets, with average densities
  ranging from 3.9 to 5.5 g/cm3, contain large
  amounts of rock and iron, have iron cores, and
  have relative element ratios similar to the Sun
  except for deficiencies in hydrogen, helium and
  other elements typically found in gaseous
  compounds
• The Jovian planets, with average densities
  ranging from 0.71 to 1.67 g/cm3, have relative
  element ratios similar to the Sun and have
  Earth-sized rocky cores
• The planets and Sun must have formed from the
  same interstellar cloud of gas and dust

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Components of the Solar System

• Age of the Solar System
• All objects in the Solar System seem to have
  formed at nearly the same time
• Radioactive dating of rocks from the Earth, Moon,
  and some asteroids suggests an age of about 4.5
  billion yrs
• A similar age is found for the Sun based on
  current observations and nuclear reaction rates
• Bodes Law The Search for Order
• Very roughly, each planet is about twice as far
  from the Sun as its inner neighbor
• This progression can be expressed mathematically
  (including the asteroid belt but not Neptune) as
  Bodes Law
• Bodes Law may be just chance or it may be
  telling us something profound astronomers do
  not know

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Origin of the Solar System

• Introduction
• A theory of the Solar Systems formation must
  account for the following
• The Solar System is flat with all the planets
  orbiting in the same direction
• Two types of planets exist rocky inner planets
  and gaseous/liquid/icy outer planets
• Outer planets have similar composition to Sun,
  while inner planets composition resembles the
  Suns minus gases that condense only at low
  temperatures
• All Solar System bodies appear to be less than
  4.5 billion years old
• Other details structure of asteroids, cratering
  of planetary surfaces, detailed chemical
  composition of surface rocks and atmospheres, etc.

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Origin of the Solar System

• Introduction (continued)
• Currently favored theory for the Solar Systems
  origin is the solar nebula hypothesis
• Derived from 18th century ideas of Laplace and
  Kant
• Proposes that Solar System evolved from a
  rotating, flattened disk of gas and dust (an
  interstellar cloud), the outer part of the disk
  becoming the planets and the inner part becoming
  the Sun
• This hypothesis naturally explains the Solar
  Systems flatness and the common direction of
  motion of the planets around the Sun
• Interstellar clouds are common between the stars
  in our galaxy and this suggests that most stars
  may have planets around them

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Origin of the Solar System

• Interstellar Clouds
• Come in many shapes and sizes one that formed
  Solar System was probably a few light years in
  diameter and 2 solar masses
• Typical clouds are 71 hydrogen, 27 helium, and
  traces of the other elements
• Clouds also contain tiny dust particles called
  interstellar grains
• Grains size from large molecules to a few
  micrometers
• They are a mixture of silicates, iron and carbon
  compounds, and water ice
• Generally, the clouds contain elements in
  proportions similar to those found in the Sun
• Triggered by a collision with another cloud or a
  nearby exploding star, rotation forces clouds to
  gravitationally collapse into a rotating disk

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Origin of the Solar System

• Formation of the Solar Nebula
• A few million years passes for a cloud to
  collapse into a rotating disk with a bulge in the
  center
• This disk, about 200 AU across and 10 AU thick,
  is called the solar nebula with the bulge
  becoming the Sun and the disk condensing into
  planets
• Before the planets formed, the inner part of the
  disk was hot, heated by gas falling onto the disk
  and a young Sun the outer disk was colder than
  the freezing point of water
• Gas/dust disks have been observed

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Origin of the Solar System

• Condensation in the Solar Nebula
• Condensation occurs when gas cools below a
  critical temperature at a given gas pressure and
  its molecules bind together to form liquid/solid
  particles
• Iron vapor will condense at 1300 K, silicates
  will condense at 1200 K, and water vapor will
  condense at room temperature in air
• In a mixture of gases, materials with the highest
  vaporization temperature condense first
• Condensation ceases when the temperature never
  drops low enough
• Sun kept inner solar nebula (out to almost
  Jupiters orbit) too hot for anything but iron
  and silicate materials to condense
• Outer solar nebula cold enough for ice to condense

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Origin of the Solar System

• Accretion and Planetesimals
• Next step is for the tiny particles to stick
  together, perhaps by electrical forces, into
  bigger pieces in a process called accretion
• As long as collision are not too violent,
  accretion leads to objects, called planetesimals,
  ranging in size from millimeters to kilometers
• Planetesimals in the inner solar nebula were
  rocky-iron composites, while planetesimals in the
  outer solar nebula were icy-rocky-iron composites
• Formation of the Planets
• Planets formed from gentle collisions of the
  planetesimals, which dominated over more violent
  shattering collisions

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Origin of the Solar System

• Formation of the Planets (continued)
• Simulations show that planetesimal collisions
  gradually lead to approximately circular
  planetary orbits
• As planetesimals grew in size and mass their
  increased gravitational attraction helped them
  grow faster into clumps and rings surrounding the
  Sun
• Planet growth was especially fast in the outer
  solar nebula due to
• Larger volume of material to draw upon
• Larger objects (bigger than Earth) could start
  gravitationally capturing gases like H and He
• Continued planetesimal bombardment and internal
  radioactivity melted the planets and led to the
  density differentiation of planetary interiors

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Origin of the Solar System

• Direct Formation of Giant Planets
• It is possible the outer regions of the solar
  nebula were cold and dense enough for gravity to
  pull gas together into the giant planets without
  the need to first form cores from planetesimals
• Formation of Moons
• Moons of the outer planets were probably formed
  from planetesimals orbiting the growing planets
• Not large enough to capture H or He, the outer
  moons are mainly rock and ice giving them solid
  surfaces
• Final Stages of Planet Formation
• Rain of planetesimals cratered surfaces
• Remaining planetesimals became small moons,
  comets, and asteroids

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Origin of the Solar System

• Formation of Atmospheres
• Atmospheres were the last planet-forming process
• Outer planets gravitationally captured their
  atmospheres from the solar nebula
• Inner planets created their atmospheres by
  volcanic activity and perhaps from comets and
  asteroids that vaporized on impact
• Objects like Mercury and the Moon are too small
  not enough gravity to retain any gases on their
  surfaces
• Cleaning up the Solar System
• Residual gas and dust swept out of the Solar
  System by young Suns intense solar wind

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Other Planetary Systems

• Evidence exists for planets around other nearby
  stars
• The new planets are not observed directly, but
  rather by their gravitational effects on their
  parent star
• These new planets are a surprise - they have huge
  planets very close to their parent stars
• Idea The huge planets formed far from their
  stars as current theory would project, but their
  orbits subsequently shrank
• This migration of planets may be caused by
  interactions between forming planets and leftover
  gas and dust in the disk